Small waterplane area ships (SWAS) generally consist of a vessel having at least one waterline, located below its design draft, with a waterplane area that is significantly larger than the waterplane area at its design draft. One form of such vessel is a small waterplane twin hull vessel (also referred to as a SWATH vessel) which generally consists of two submerged hulls, originally formed of uniform cross section, connected to a work platform or upper hull by elongated struts which have a cross section along any given waterplane area that is substantially smaller than a waterplane area cross section of the submerged hulls. Thus, at the design waterline, with the hulls submerged, such vessels have a small waterplane area.
SWAS vessels may have one or more lower hulls connected to the work platform or super structure by one or more struts. Originally, SWAS vessels utilized single struts between two submerged hulls and the upper platform, as shown for example in U.S. Pat. No. 3,447,502 issued to Lang, and U.S. Pat. No. 4,552,083 issued to Schmidt. Some time ago, however, the Naval Ocean System Center at San Diego and Honolulu developed a SWAS design characterized by having a least two struts associated with each submerged hull. These vessels are further characterized by submerged twin hulls with uniform cross sections and at least two narrow struts making a connection, at the forward and aft ends of the submerged hulls and the platform. These struts typically extend vertically, as shown for example in U.S. Pat. Nos. 3,623,444 and 3,897,944, issued to Lang. Other forms of such vessels have been disclosed which contain a single lower hull connected by one or more struts to the work platform and vessels having three or more lower hulls connected to the work platform by one or more struts associated with each hull. Other vessels of this general type are also disclosed in U.S. Pat. No. 4,557,211 and Japanese Patent No. 52,987 issued Jan. 11, 1977.
SWAS vessels of this type usually include sponsons (alternatively referred to in the art as upper hulls or upper struts) which are structures positioned above the struts and below the work platform or super structure that have significantly increasing waterplane areas extending from the strut to the platform. That is, these sponsons are flared hull type structures in cross section having deadrises extending along the length of the vessel. The sponsons may be continuous or segmented over each strut. The struts themselves are generally foil shaped and constant in cross sectional areas. However, as is known in the art, these struts can also be tapered and/or can be canted at negative or positive dihedral angles.
In SWAS vessels, it is desirable to maintain a minimum waterplane area at the design waterline for most efficient operation of the vessel. However, this desirable goal is limited by the need for a minimum waterplane area required to maintain hydrostatic stability. As a result, existing SWAS vessels commonly have a problem with trim and heel stability due to the small waterplane area of the struts. These vessels also suffer from high frictional drag due to relatively large surface areas formed by the struts despite every effort that has been made to minimize this.
Previously proposed semi-submerged vessels use an arrangement of elongated (small cross-sectional area to length) submerged hulls to provide the majority of the buoyancy. For efficient operation from the standpoint of powering and fuel consumption, SWATH, as with all displacement ships, are presently limited in speeds to those having a Froude number of less than 0.4.
Froude number (F) is defined as follows: ##EQU1## wherev=speed
g=acceleration due to gravity PA1 l=length of hull. PA1 "as long a length as is compatible with other design requirements," Principles of Naval Architecture, Comstock, p. 345; PA1 "greater length will reduce wave-making resistance but increase the frictional resistance," Comstock, p. 342; and PA1 "vessels . . . are made as long and slender as practicable," Hoerner, p. 11-12.
The limit in speed of a displacement ship is best described in Modern Ship Design, by Thomas C. Gillmer, 1970 which states, "The practical limiting speed for displacement surface vessels is basically that of wavelength to ship length, where one wavelength, created by the ship, is equal to the ship's waterline length.
This, expressed quantatively, is V/.sqroot.L.apprxeq.1.3 (or F=0.39), and V is sometimes called the hull speed. When a surface ship attempts to exceed this speed it finds itself literally climbing a hill that it is creating. In exceptional cases of slim, highly powered ships such as destroyers, it is possible to exceed this speed, but it is seldom profitable."
The limitation in speed is primarily due to the large increase in wave resistance that occurs between a Froude number of 0.4 and 0.8. This increase in wave resistance is well established in the prior art for all surface displacement ships and is often referred to as the resistance or powering "hump." See Fluid-Dynamics Resistance, by Sighard F. Hoerner, 1965. Because of the high wave resistance, operation in the "hump" speed region results in high propulsion power and inefficient fuel usage. According to Gilmer, supra, "A ship may be required to maintain a constant operational speed for long periods and it is clearly desirable that it should not do so at a hump on the Cw (wave drag) curve" (pg. 160). Normal operation for a displacement ship is at a Froude number corresponding to a "hollow" in the wave drag curve at a Froude number lower than the primary hump. The operational Froude number for various ship types is shown in FIG. 5.22 of Mechanics of Marine Vehicles, Clayton and Bishop, p. 220 and table A page 11-15, Hoerner, supra. Only the destroyer with its abundance of power operates at a Froude number above 0.4.
To delay the onset of high wave making resistance the prior art calls for:
Operation at a Froude number greater than 0.8 substantially reduces wave resistance. "The pressure distribution about a high speed vehicle is therefore quite similar to that about a vehicle progressing at a very low speed . . . This means that the wave making resistance of high speed vehicles (Fr.gtoreq.1.5, say) is small as it is for vehicles operating at very low speeds (Fr.ltoreq.0.15, say)" Clayton and Bishop, p. 219; however, to exceed the "hump" speed region requires excessive propulsion power for displacement (including SWATH) ships of the conventional form.
Recently it has been found that a small waterplane area hull form which operates at reduced wave resistance and permits efficient operation to high speeds, that is, where the Froude number is greater than 0.8, can be provided using streamlined struts and streamlined foils extending transversely between the struts which have a significantly reduced stream wise length, when compared to elongated hulls of the conventional design. This arrangement will effectively increase the Froude number at a given speed to a Froude number at which no conventional displacement ship operates. It allows SWATH and SWAS vessels to operate at higher speed while retaining their characteristic low motions in a seaway. This is accomplished through reduced wavemaking drag at high speeds.
Two additional concepts that have been advanced to achieve high speeds with good seakeeping are a hybrid SWATH hullform, or HYSWATH and a hybrid catamaran hull form, or hycat (or foilcat, catafoil or hysucat). Both concepts attach one or more hydrofoils to the underwater hulls. At rest and at low speeds these vessels' struts or catamaran hulls are immersed to a relatively deeper draft to maintain sufficient submerged volumes to buoyantly support the vessel. Above certain critical speeds the hydrofoil(s) generate sufficient hydrodynamic lift to partially raise the vessel to a shallower draft. The partial lifting of the vessel raises the struts or catamaran hulls along their entire waterline length to a shallower draft raising previously submerged sections out of the water, thus reducing the wetted surface area frictional drag. The raising of the struts or catamaran hulls to a shallower draft further reduces residual resistance by reducing the amount of submerged volume and cross sectional area of struts and catamaran hulls which are generally tapered or flared (V shaped cross sections). The amount of dynamic lift of the hydrofoil(s) is a design variable that ranges from 30% to 90% of the vessels full load displacement.
In catamaran, trimaran and monohull SWAS configurations, the buoyant submerged hulls are oriented longitudinally, that is the submerged hull's length is greater than its width. Since the vessel's longitudinal center of gravity and buoyancy is usually at midships, this creates a large moment arm for any forces acting on the ends of the submerged hull(s). This condition exists when the vessel is in sea conditions where there is a relatively long wavelength compared to the ship's length such as when the vessel is at rest or is running in following seas. Under these conditions, the wave forces acting on the ends of the submerged hulls can give rise to significant motions. In addition, prior hull forms discussed thus far have the vessel's waterplane areas distributed longitudinally and transversely to provide required flotation to maintain hydrostatic stability. The waterplane areas of the water piercing struts or hull sponsons are typically vertically aligned above the vessels buoyant submerged hulls. The vessel's center of buoyancy is necessarily aligned with the vessels center of gravity and the typical arrangement of the waterplane area also results in alignment of the center of flotation.
It is an object of the present invention to provide an improved SWAS vessel which can operate efficiently at high speeds.
Another object of the invention is to provide a SWAS which has higher propulsive efficiency as compared to the prior art.
Yet another object of the present invention is to provide a SWAS vessel with a higher deadweight to lightship ratio as compared to the prior art.
A further object of the invention is to produce a SWAS vessel with reduced structural loads, a low wake at high speeds and improved control of motions.